Intelligence transmitting circuit



July 16, 1940.

R BLACK. JR

INTELLIGENCE TRANSMITTING CIRCUIT Filed May 19, 1938 FIG. I

TO LINE FIG 2 NO RECTIFIER IN CIRCUIT SMALL SHUNT ACROSS RECTIFIER LARGESHUNT ACROSS RECTIFIER RECTIFIER ALONE IN CIRCUIT sou/v0 WAVE PRESSUREINI/ENTOR R. BLACK, JR.

ATTORNEY Patented July 16, 1940 7 UNITED STATES PATENT rrlclaINTELLIGENCE TRANSMITTHNG CIRCUIT Robert Black, Jr., Maplewood, N. 3.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application May 19, 1938, Serial No. 208,770

13 Claims.

is known, include a transmitter or microphone. having a vibratileelement, such as a diaphragm,

responsive to pressure waves, the transmitter or microphone functioningto translatesound waves into corresponding voltage variations in thetelephone circuit. The magnitude of such variations is generallydirectly dependent upon the intensity or pressure of the waves actingupon the vibratile element or diaphragm, throughout a fairly wide rangeof pressures so that faithful translation of speech may be obtained by anumber of users of the same instrument and for different intensities ofspeech. However, in some installations, such as in central oflices, thetransmitter is operated in a field of extraneous pressure waves,attributable to room and other noises, which produce correspondingvariations in the voltage across the output terminals of the circuit.These variations may interfere with the intelligibility of the speechbeing transmitted and may result in low fidelity and unfaithfultransmission. Consequently, entirely clear communication between theoperator and, for example, a subscriber may not be attained. Inaddition, such variations may introduce objectionable side tone effectsin the operators circuit and thereby interfere with reception by theoperator of speech over the telephone line. 4 I

One general object of this invention is to improve the intelligibilityof speech transmission in telephone circuits.

More specifically, one object of this invention is to discriminatebetween sound waves corresponding to the speech to be transmitted andother waves attributable to extraneous noises whereby the sound waveswill be translated with high fidelity and waves attributable to roomnoises will be materially or entirely suppressed.

In accordance with one feature of this invention, a device having anon-linear resistance characteristic, such as a copper oxide rectifierelement, is associated with the transmitter so that the translation orinput-output characteristic of the circuit is altered and the efficiencyof translation at pressures corresponding to undesired and extraneousnoises is reduced.

In accordance with another feature of this invention, means, such as adevice having a substantially linear resistance characteristic, isconnectedin circuit with the rectifier element for allowing alterationof the input-output characteristic of the circuit to obtain mostfaithful translation of speech and maximum suppression, in effect, ofextraneous noises, for the particular locations at and thepart'icularconditions under which the transmitter is operated. i

The invention and the foregoing and other features thereof willbeunderstood more clearly and fully from the following detaileddescription with reference to the accompanying drawing in which: l

Fig. l is a circuit diagram illustrating an operators circuitillustrative of one embodiment of this invention; and" Fig. 2 is a graphshowing typical translation characteristics obtainable, in accordancewith this invention, for the operators circuit illustrated in Fig. 1.

Referring now to the drawing, the telephone operators circuit shown inFig. 1 comprises a transmitter mesh A and a receiver mesh B, including 0and coupled byan induction coil having three windings I0, I l and i2.The receiver mesh B has terminals l3 adapted to be connected to atelephone line, and includes a receiver l4 connected across the windingl2,a condenser I 5 in series with the windings l l and I2, and onewinding iii of a repeater coil, connected in shunt with the receiver 14.The receiver I l may be of any well-known type and construction.

. The transmitter mesh A comprises a transmitter IT, a source, such as abattery l8, and a suitable retardation coil l9, defining a seriescircuit with the winding 10. The transmitter may be of any type andconstruction. For example, it may be a carbon granule transmitter of thegeneral construction disclosed in Patent 2,042,822, ranted June 2, 1936,to Arthur F. Bennett and William L. Tuffnell.

When the vibratile element or diaphragm of the transmtter I! is'actuatedby sound waves, corr-e- 40 sponding variations in the voltage across thewinding ill, and hence across the output terminals l3, are produced, themagnitude ofthe variations being dependent upon the intensityof thesound Waves. In a particular and typical operators circuit, the voltageacross the terminals [3 varies substantially linearly with wave pressureas indicated by the line C in Fig. 2, wherein the abscissae representsound wave pressures and the ordinates represent voltages across theoutput terminals [3. The slope of the line C is substantially unity.That is, they line is at substantially 45 degrees, so that substantiallyequal increments in voltage resuit from similar increments in sound wavepressure, throughout a wide range of pressures. l

In some installations, as noted heretofore, for example, in centralofiices, the transmitter is operated in a field of sound wavesattributable to extraneous and room noises. Some of these waves act uponthe diaphragm or vibratile element of the transmitter I! and producecorresponding variations in the potential across the output terminals93. These variations alone or superimposed upon the variations resultingfrom speech waves translated by the transmitter may produceobjectionable disturbances in. the telephone line, cause distortion inthe speech translated, and hence may result in a decrease in the qualityof the speech transmitted. It has been ascertained that generally mostof the sound waves attributable to extraneous and room noises and resulting in the effects noted above are of relatively low pressures ascompared with those ordinarily and usually produced by a person speakinginto the transmitter.

In accordance with this invention, the objectionable effects ofextraneous and room noises are overcome in a large measure by. alteringthe input-output characteristic of the circuit so that, in effect, adiscrimination between such noises and the sounds to be transmitted isobtained. In one illustrative embodiment, shown in Fig. 1, a devicehaving a non-linear resistance characteristic isconnected in circuit witthe transmitter for thus altering the input-oifisput characteristic.

Specifically, this device may comprise one or more dry disc rectifierunits 20, such as coppercopper oxide rectifier units, the resistance ofwhich, as is known, decreases as the potential across the terminalsthereof increases. As illustrated in Fig. 1, two rectifier unitsconnected in opposingmultiple and across the source l8 and coil l9, maybe employed. A condenser 2i preferably is connected in series with therectifier units in order to prevent the flow of a direct current throughthe rectifier.

It has been found that when suitable rectifiers are thus connected, asshown by the line C1 in Fig. 2, the input-output characteristic of theoperators circuit includes two portions, X1 and Y1, having markedlydifferent slopes. For example, at low pressures the slope of thecharacteristic, as indicated by the portion Y1, may be substantiallyunity or less than unity whereas at higher pressures, as indicated bythe portion X1, the slope is greater than unity. Consequently, athigherpressures, which are those most generally prevalent as produced by thesound to be translated, the voltage variations across the outputterminals l3 are considerably greater per unit of pressure increment,than the variations corresponding to equal increments in the lowpressure range, wherein, as noted heretofore, the Waves attributable toextraneous and room noises are found. Hence, in effect, a discriminationbetween the desired sound pressures and the undesired noise pressures isobtained. 1

. It will be noted also, by comparison of the lines C and C1, thatalthough the use of the rectifier units results in some decrease in theefficiency of the circuit, the decrease is much more pronounced at thelower pressures than at the higher.

The decrease in efiiciency and also the extent of the range of pressuresagainst which discrimination is effected may be controlled in accordancewith the invention by providing a linear re sistance element 22 in shuntwith the rectifier units. The effects of a shunt linear resistance uponthe input-output characteristic are illustrated by the lines C2 and C3in Fig. 2, each of which includes a substantially linear portion (Y2 orY3) of substantially unity slope and a slightly concave upward portion(X2 or Y3) having a slope greater than unity. In the specific circuitsutilized, the shunt linear resistance resulting in the characteristicillustrated by the line C2 was fairly large (of about 450 ohms) and thatresulting in the characteristic illustrated by the line C3 was smaller(of about 200 ohms).

From these lines, it will be seen that the magnitude of the shunt linearresistance 22 affects the eificiency of the circuit and also determinesthe slope of the upper portion of the characteristic so that it allowsdetermination of the range of pressures against which discrimination isdesired. The range of pressures corresponding to extraneous and roomnoises will vary with locations of the transmitter. The resistance 22may be variable and adjusted so that maximum suppression, in eifect, ofthe noises at any particular location is attained.

Although a specific embodiment of the invention has been shown anddescribed, it will be understood, of course, that various modificationsmay be made therein without departing from the scope and spirit of thisinvention as defined in the appended claims. For example, although thenon-linear resistance device has been described as comprisingcopper-copper oxide rectifier units, other devices having similarproperties may be employed.

What is claimed is:

1. A telephone circuit comprising a series circuit including a telephonetransmitter of the carbon granule type, a battery and an output coil,and a branch circuit including a condenser and a non-linear resistancein series, said branch circuit having one end connected to a pointbetween said transmitter and said battery and the other end connected toa point between said battery and said coil.

2. A telephone circuit comprising a closed series circuit including atelephone transmitter and a biasing source for said transmitter, and acircuit in shunt with said source including a blocking condenser and apair of rectifiers connected in opposing multiple and in series withsaid condenser.

3. A telephone circuit in accordance with claim 2 comprising a linearresistance in shunt with said rectifiers.

4. In an operators circuit, a closed transmitter mesh comprising anoutput coil, a telephone transmitter and a two-way non-linear resistanceall connected in series, a source of current for said transmitterconnected across points in said mesh on opposite sides of saidresistance, and a blocking condenser between said source and saidresistance.

5. In an operators circuit, a transmitter mesh as defined in claim 4including a linear resistance in shunt with said non-linear resistance.

6. An intelligence transmitting circuit comprising a mesh including atransmitter and an output element, said transmitter being operable totranslate sound waves into electrical impulses and thereby to producevariations in the voltage across said element, means for decreasing theefilciency of said transrnittenas determined by relative voltage changesacross said element with respect to sound wave pressures at saidtransmitter, non-uniformly throughout a range of sound wave pressures,the decrease being greatest throughout a range of low wave pressures,and means for varying the decrease in eificiency.

7. A telephone circuit comprising a transmitting circuit including anoutput element and a transmitter operable to translate sound waves intovoltage variations across said output element, said transmitter having arising wave pressure-output voltage characteristic, the output voltagebeing measured across said element, means in circuit with saidtransmitter for increasing the slope of said characteristic through aportion thereof without altering the slope of a lower portion thereof,whereby substantial discrimination between pressures corresponding tosaid portions is efiected, and means for altering the slope of saidfirst portion.

8. In an operators circuit, a transmitter mesh including a transmitterin series with a pair of unidirectional non-linear resistances connectedin opposing multiple.

9. In an operators circuit, a transmitter mesh in accordance with claim8 comprising a linear resistance in shunt with said non--linearresistances.

10. In an operators circuit, a transmitter mesh including an outputwinding, a source of current and a transmitter in series, and a shuntpath across said transmitter and said winding including a pair of copperoxide rectifiers connected in opposing multiple and a blocking condenserin series therewith.

11. In an operators circuit, a transmitter mesh in accordance with claim10 comprising a linear resistance directly in shunt with saidrectifiers.

12. In an operators circuit, a series circuit including a transmitter, asource of current and an output element, and a branch circuithaving oneend connected between said transmitter and said source and the other endconnected between said source and said element, said branch circuitincluding a pair of unidirectional non-linear resistances connected inopposing multiple.

13. In a telephone operators circuit, a circuit including a transmitter,a source of direct current and a winding of an induction coil seriallyconnected, and a branch circuit having one end connected between saidtransmitter and said source and the other end connected between saidsource and said coil, said branch circuit including a unidirectionalnon-Iinear resistance and a linear resistance in shunt thereto.

ROBERT BLACK, JR.

